In the present day, freight has been transported rapidly over the world with the development of transportation means such as aircraft. However, in the case of oil, natural gas, vehicles, and containers with a very large freight volume and a heavy freight weight, they could not be transported in great volumes at a time by the aircraft and, therefore, it is common to transport them by a ship.
In the event that freight is transported by using a ship, it is required to make the ship large and move at high speed in order to transport a great amount of freight at a time and rapidly. However, hull vibration is increased by a ship propeller and a lot of fuel is consumed due to an increase of an engine horsepower resulting from the large ship moving at a high speed.
Therefore, there is a need for the development of a device which can reduce hull vibration by the ship propeller and save fuel even when the horsepower of the engine is increased.
FIG. 1 is a schematic side view of a conventional fin device of a ship. FIG. 2 is a schematic side view illustrating the flow of a fluid, which is controlled by the conventional fin device of the ship. FIG. 3 illustrates the comparison of the speed of a fluid flowing into the propeller of the ship provided with the fin device shown in FIG. 1 with the speed of a fluid flowing into the propeller of a bare hull provided with no fin device. FIG. 4 illustrates the comparison of constant pressure lines of the ship provided with the fin device shown in FIG. 1 with constant pressure lines of the bare hull.
The conventional fin device of the ship is disclosed in Japanese Patent Laid-Open Publication No. 2002-362485, and was contrived to improve propulsive efficiency and reduce resistance of a ship body.
The fin device of the ship includes two strap fins 5 and 6 which are respectively provided on the front and rear sides. Both the fins 5 and 6 are mounted to an outer plate of the ship body so as to protrude at an almost right angle, and have a thin thickness.
The front fin 5 has an installation starting point at a location of a distance S (within 15% of Lbp) on the prow side from a vertical line 8 of the stern, and is installed under the central height of a propeller 4. The front fin 5 is inclined such that its height from the bottom of the ship increases as it goes toward the stern. The front fin 5 has a length L1 smaller than the diameter D of the propeller 4. A protruding width of the front fin 5 from the ship body is smaller than 10% of the diameter D of the propeller 4.
The rear fin 6 is disposed in parallel to the bottom of the ship between the centerline of the propeller 4 and a propeller tip, and is installed right ahead of the propeller. The rear fin 6 has a length L2 smaller than the diameter D of the propeller 4. A protruding width of the rear fin 6 from the ship body is smaller than 20% of the diameter D of the propeller 4.
The front fin 5 serves to weaken a vortex (bilge vortex) 9 which spirals from the bottom of the ship to the side of the ship, and also sequentially guide the vortex toward the propeller. The rear fin 6 serves to prevent diffusion of the bilge vortex 9 which is guided toward the propeller 4 by the front fin 5. The flow of a fluid 10, which flows through a gap between the front fin 5 and the rear fin 6, serves to prevent diffusion of the bilge vortex 9.
If the bilge vortex 9 is weakened as described above, the fluid flown into the propeller becomes more uniform. If diffusion of the bilge vortex 9 is prevented, induction resistance caused by the bilge vortex 9 is decreased. Thus, resistance of the ship body can be reduced and propulsive efficiency of a ship can be improved.
The present inventors performed a numerical analysis in order to confirm the conventional effects. The results of the numerical analysis are shown in FIGS. 3 and 4.
FIG. 3(a) shows the speed of a fluid flowing into the propeller of a bare hull provided with no fin device, and FIG. 3(b) shows the speed of a fluid flowing into the propeller of a ship provided with the fin device shown in FIG. 1. In FIG. 4, blue color shows the constant pressure lines of the bare hull, and dark color shows the constant pressure lines of the ship provided with the fin device shown in. In FIG. 4, the closer toward the stem, the larger the constant pressure line.
The present inventors set an attachment condition of the fin within a range of the embodiment disclosed in Japanese Patent Laid-Open Publication No. 2002-362485 in performing the numerical analysis.
The front fin 5 was disposed at the location of 15% of Lbp from the perpendicular line A.P. of the stern in the length direction of the ship and mounted at the location of 30% of the diameter of the propeller from the bottom of the ship in the height direction of the ship. Further, the length of the front fin 5 was set to the same as the propeller diameter, the width of the front fin 5 was set to 7% of the propeller diameter, and an angle of the front fin 5 to the bottom of the ship was set to 10 degrees. Furthermore, the rear fin 6 was mounted right in front of the propeller in the length direction of the ship, and at the location of 90% of the propeller diameter from the bottom of the ship in the height direction of the ship. The length of the rear fin 6 was set to 80% of the propeller diameter, the width of the rear fin 6 was set to 10% of the propeller diameter, and the rear fin 6 was set in parallel to the bottom of the ship.
When performing a numerical analysis under the above conditions, it can be seen from FIG. 3 that there is almost at the lower side of the propeller in the speed of a fluid flowing into the propeller of the ship provided with the fin device shown in FIG. 1 compared with the speed of a fluid flowing into the propeller of the bare hull provided with no fin device. It can also be seen that there are speed-reduced portions (portions in which light blue was changed to deep blue) at the upper side of the propeller. It means that the effect of reducing vibration by the propeller rarely appears.
Further, from FIG. 4, it can be seen that the constant pressure lines of the ship provided with the fin device shown in FIG. 1 are almost identical to those of the bare hull and, therefore, pressure resistance is rarely decreased. It can also be seen that propulsive efficiency of the ship is not much improved since pressure resistance is not reduced as described above.